Define and implement task for collecting MET stat info in a more usable form

[maddenp-cu]

Currently, the stats task just produces .stat and .txt files from MET grid_stat and point_stat runs. The plots task works with these files directly, but users with advanced plotting needs will want to access the stats data in a more fluent way. Add a new top-level, public task to perform this.

Some notes:

• netCDF might be a good format; what follows will assume netCDF. Consult with would-be users about design.
• The task for this netCDF file should yield requirements for the stats task(s) that create the output that it will then package.
• It should be possible to augment the netCDF file. For example, if a user runs a range of cycles, then adds a new cycle to the YAML config, the new stats should be produced, then appended to the netCDF. This will require the ready predicate for the task to test file content, not just existence.
• Removing data from the netCDF is out of scope: If the user removes cycles, leadtimes, etc. from the config, the task need not remove data from the netCDF file.
• Consider simultaneously revamping the plots task to require this new task and then consume its output: The .stat / .txt parsing code currently used for plotting could then move to the new task, where it would otherwise probably have to be reimplemented anyway. This refactor would also provide a valuable internal use case for the netCDF, helping to ensure that it is properly maintained.